1. Field of the Invention
The present invention relates generally to a method and an apparatus for printing graphics, and in particular to the printing of documents containing transparent graphics.
2. Description of Related Art
The most common purpose for which present day personal computers are used is word processing. A word processing application allows the user to create text documents, to edit text documents, and to print text documents on a printer.
Most word processing applications offer the possibility to import graphics into a text document, or to generate graphics in a text document. These graphics may be transparent graphics, which means the graphics may be transparently superimposed on another object of the text document, such as some text or any other graphics object. The most common way to represent such transparency information is the so-called alpha channel, which uses the uppermost eight bits of thirty-two bits for each pixel to represent the transparency information.
In document 100, shown in FIG. 1, the graphics objects 110 and 120 are transparent graphics objects. Text object 130 overlaps transparent graphics object 110 and text object 150 overlaps transparent graphics object 120. Text object 140 does not overlap any transparent graphics object.
To ensure an acceptable printing quality, according to the prior art, text documents containing transparent graphics objects, such as text document 100, had to be printed as bitmap objects. This means that the whole page of a document containing transparent graphics objects had to be converted into a bitmap for printing on a printer. This was necessary to ensure that the text elements, which might overlap with the transparent graphics objects, were of acceptable printing quality. Converting the whole page of a text document into a bitmap was performance and memory consuming.